Phase change memory (PCM) is a type of non-volatile computer memory that utilizes differences in the electrical resistivity of the crystalline and amorphous phase states of memory materials. Devices that incorporate PCM typically comprise substrates on which a particular memory material (e.g., a chalcogenide) is deposited. The memory material is typically deposited within structures (such as holes, trenches, or the like) in or on the surfaces of the substrate. Patterned electrodes are also deposited on the substrate to allow for the conduction of current. The conduction of current is effected through the deposited memory material, with the level of current being dependent on the resistivity and heating efficiency of such memory material and its alloy properties on phase change.
One memory material used in the manufacture of PCM devices is germanium antimony telluride (GST). The GST materials can function in principle very effectively as phase change material for a volume, v, having characteristic dimensions as small as 5 nm. The trend is to make PCM devices based on GST with characteristic dimensions in the regime of 30 to 10 nm or less in future generations of devices. Also, to confine the heat for phase change, a hole structure with dielectric surrounding the hole is highly preferred, with the aspect ratio of the hole being greater than 1, typically greater than 3:1 to improve heating efficiency. The deposition of germanium antimony telluride by chemical vapor deposition (CVD) process(es) can be carried out to produce a CVD film of amorphous phase or limited crystallinity. The deposition of germanium antimony telluride is difficult to achieve because non-perfect conformality (<100%) or smooth surfaces will leave voids deep inside the hole. This is because GST is deposited faster on the upper portions of the wall of the hole. The hole as a result may be filled at the upper part of the hole and occluded from deposition on the lower surfaces in the hole. Even if the deposition is 100% conformal, any non-smooth surface due to the protrusion of GST as the result of locally enhanced growth, especially local crystalline growth that is typically faster than the amorphous growth that provides the best conformality, will lead to a “seam” in the GST filled structure where the films on the side walls of the hole or trench meet. This protrusion-like growth is readily formed for GST materials with high levels of crystallization or at low crystallization temperature, which provides a faster phase change alloy for PCM. Furthermore, as desired device performance in terms of smaller reset current and higher speed is realized, the cross section of the hole becomes smaller and the height becomes larger (illustrative hole geometries involving, for example, holes less than 20 nm in diameter or equivalent diameter but with depths of greater than 30 nm). The manufacturing of these hole structures (less than 20 nm) is costly and technologically challenging because it is difficult to fabricate small holes with precision control, and high aspect ratio small holes are difficult to attain for ion etch processes, as ions become difficult to transport into the small and deep holes during the etching process.
For smaller structures of 10 nm diameter, in order to keep the cross-sectional area of the hole variation to about 10%, a 10 nm feature has an approximately 5% diameter variation, which is 0.5 nm and is close to the molecular size of lithography resist. In comparison, maintaining a 30 nm diameter with 10% variation of cross-sectional area requires about 1.5 nm diameter control, which is more readily achieved. The coated GST film thickness on the wall of the hole is controlled by conformal deposition of GST, independent of the lithographic process, and is typically about 1% of the deposition thickness regardless of the absolute thickness of the film, which is very easy to control. Moreover, sufficiently filling smaller holes (whether from starting as a small hole about 5 nm or 10 nm in diameter with dielectric surrounding it, or during the latter stage of GST deposition in filling a hole of 30 to 100 nm, the small holes in these two cases having large aspect ratios) has always been challenging due to problems associated with molecular transport into small and deep holes. The cross-sectional area is 2πD·t, where D is the diameter and t is the thickness, so the thickness is linearly related to the D variation. The hole depth of the PCM device is continually increasing in further device development, adding additional difficulty.